Gears are in frequent use in transmissions, etc. for automobiles. In recent years, a further improvement in the machining accuracy of the gears has been desired with the aim of reducing vibrations and noise of the transmissions. With the gear machining method, it is common practice to carry out gear cutting in a predetermined gear material, thereby forming a gear, heat-treat the gear formed by gear cutting, and then perform finishing (grinding) in order to remove strain or the like due to this heat treatment. So far, the gear to be machined after heat treatment and a grindstone have been put in mesh, with an angle of axis intersection being given, and their grinding has been carried out. Tools used for these grinding processes are available in shapes, such as an external gear shape, an internal gear shape, and a screw (worm) shape, in accordance with the shape of the gear to be ground.
The sharpness of the grindstone decreases as the grindstone wears with the progress of grinding. After a predetermined number of gears are ground, therefore, it becomes necessary to dress the grindstone having a worn-out grinding surface, thereby regenerating a sharp cutting edge surface.
As a method for dressing, it is performed to dress the grindstone with the use of a dressing gear set to have nearly the same gear specifications as those of the finished gear. During dressing, it is general practice to liken the dressing gear to the gear to be machined, set the angle of intersection of the axes of the dressing gear and the grindstone at the angle of axis intersection during grinding of the gear to be machined, and carry out dressing at this angle. Patent Document 1, on the other hand, discloses a hard gear honing method related to grinding of an externally toothed work (external gear to be machined) with respect to an internally toothed grindstone, with a crossed axes angle (angle of axis intersection) between an externally toothed dressing gear and the internally toothed grindstone being gradually decreased each time dressing is performed, in order to increase the number of dressings becoming possible, and lengthen the life of the grindstone.
PRIOR ART DOCUMENTS
Patent Documents
Patent Document 1: JP-A-11-138346 (see, for example, the paragraphs [0011] to [0020])